The present invention relates generally to electronic ballasts for gas discharge lamps. More particularly, the present invention pertains to controlling power signals in an electronic ballast. Even more particularly, this invention relates to controlling transient current signals in the electronic ballast by monitoring the voltage across a buck inductor.
Transient current signals occur in electronic ballasts for a variety of reasons. For example, bypass capacitors associated with a power converting circuit can quickly discharge during a change in the polarity of the power signal, caused by one switching transistor ceasing to conduct while another switching transistor begins to conduct. This discharge can result in consequential, although ephemeral or transient, large current signals in the ballast. These signals can have deleterious effects, as discussed below.
Regardless of the particular source, transient currents in the power converting circuit are uniquely troubling because significant inefficiencies can result if large transient currents are present. Moreover, large transient currents in the power converting circuit can damage components or cause components to operate unreliably—both of which, intuitively, are undesirable.
Monitoring and controlling power signals in an electronic ballast has not been an undertaking ignored by the prior art. Consider U.S. Pat. No. 6,479,949 which discloses, in relevant part, an electronic ballast having a power regulator that samples the current in the power converting circuit. In response to under or overvoltage conditions in the lamp, which are reflected in the sampled current, the power regulator shifts the operating frequency of the power converter circuit relative to the resonant frequency of the LC tank circuit. This frequency shift, either away or toward the resonant frequency of the tank circuit, alters the magnitude of the driving voltage applied across the lamp to compensate for changing conditions in the ballast. However, such power control methods have several drawbacks, such as operating the power converter circuit at a frequency disparate from the resonant frequency of the tank circuit can result in power inefficiencies and difficulty effectively managing transient current signals.
What is needed, then, is an electronic ballast that can monitor and control current signals in the power converting circuit, especially transient current signals, to prevent operational inefficiencies, damage during high transient current conditions, and ineffective operation during low power conditions.